Nondestructive testing is a procedure for determining the quality or characteristics of a structure without permanently altering the structure or the structure's properties. Examples include ultrasonic and radiographic inspection. In the air transport industry, nondestructive testing of aircraft components is done to insure the structural integrity of the aircraft. In typical nondestructive testing schemes, a certified inspector performs one or more nondestructive tests on the aircraft. This process may be repeated at regular intervals to monitor and manage the structural health of the aircraft by providing, if needed, appropriate repairs.
While this type of nondestructive testing scheme can be effective, it has several drawbacks. First, the tests are typically conducted by trained inspectors, which can incur significant costs and potential operational delays especially if the inspection is part of an unscheduled maintenance action. Second, to enable efficient analysis, such as trending to identify potential problems and monitor performance, accurate, and context sensitive methods capable of making repetitive objective comparison over time are sometimes used. This process may use detailed inspection data, including inspection method parameters, damage location, decision criteria, and material properties within the context of the local structural area being inspected. Current inspection approaches generally do not preserve these necessary components with sufficient detail or accuracy
To resolve some of the above-noted drawbacks of current nondestructive schemes, other structural health management (SHM) schemes have been developed. In one structural health management technique, ultrasonic transducers are installed on the inside surface of the aircraft fuselage. The ultrasonic transducers are coupled to an onboard computer that is used to run nondestructive tests. This system allows nondestructive testing to be conducted without having an inspector bring equipment to the aircraft. The intent is to obtain real-time or on demand data from the sensors in order to detect various undesired conditions. In addition to the potential for improvements in performance, this system can reduce total operational costs by averting unscheduled maintenance, reducing the cost and number of potentially difficult inspections, and shifting line maintenance checks to base maintenance checks, while enhancing higher levels of aircraft availability.
Establishing a practical aircraft SHM system requires that the system achieve maintenance credit in an economically viable way while meeting the requirements of certifying agencies such as the Federal Aviation Administration (FAA) and Joint Aviation Authorities (JAA). There are two general aspects that need to be considered. The SHM system should be capable of achieving an airworthiness certification as part of an on-board system and should be capable of achieving approval as part of an airline maintenance program.
For each eligible aircraft that is registered in the United States, the FAA has the responsibility of issuing a Standard Airworthiness Certificate indicating that the FAA has determined the aircraft conforms to FAA-approved type design and is in a safe operating condition where appropriate. Various types of certificates and production approvals support the Standard Airworthiness Certificates. Any system used on commercial aircraft today must be certified that the system design meets certain safety requirements in performing its function. For example, Title 14 of the Code of Federal Regulations (CFR), Part 25, entitled “Airworthiness Standards: Transportation Category”, specifies that equipment, systems and installations must be designed such that the occurrence of any failure condition that would prevent the continued safe flight and landing of the aircraft is extremely improbable. Various means of showing compliance with these requirements are discussed in supporting documents such as Advisory Circulars e.g. AC 25.1309-1A, (System Design and Analysis, Advisory Circular, FAA) and documents published by industry standards groups such as SAE ARP4761. For structural health management systems this entails providing information that the portion of the structure being monitored is flight worthy.
These safety requirements for functional integrity are often expressed as the probability of an undesired effect occurring. These probabilities require analysis of the system and are based in large part upon the failure rate of the electronic components and upon the configuration of the system. Using traditional avionics system approaches, one way to meet the functional integrity requirements are for the inspection system, e.g. sensors, conversion circuitry, and processors, to be involved in some sort of voting scheme. That is, two or more independent measurements and calculations are made and the results voted such that both must provide the same answer within some reasonable tolerance. Many versions of such schemes exist; however, they require redundant sensors and electronics plus voting components in order to obtain independence. In a structural health management system, which may include numerous sensors and related equipment, the duplication of components can make the system financially prohibitive.
The above description of the safety of the system addresses the components and their configuration within a structural health monitoring system architecture. Independently, the ability of the sensors and associated detection algorithms must themselves be proven capable of detecting the structural damage to the same probabilities as described above. This is similar to proving that aircraft control laws can control an aircraft; this must be proven independently of whether or not the architecture that implements the control laws is safe.
In addition to the oversight of design and manufacture of aircraft, certification agencies also manage aircraft safety through operation and maintenance rules for air carriers and repair facilities. Modern air transport vehicles are designed to meet continued structural airworthiness provided that the airframe structural integrity is maintained by an effective inspection and corrective maintenance program. As part of these requirements a maintenance facility must ensure that all test and inspection tools used to determine the airworthiness are calibrated to standards and at intervals acceptable to the FAA. What is needed is an SHM design and approach ensuring that the occurrence of any failure condition capable of preventing the continued safe flight and landing of the aircraft is extremely improbable.